Edge termination for silicon power devices

ABSTRACT

A silicon semiconductor die comprises a heavily doped silicon substrate and an upper layer comprising doped silicon of a first conduction type disposed on the substrate. The upper layer comprises a well region of a second, opposite conduction type adjacent an edge termination zone that comprises a layer of a material having a higher critical electric field than silicon. Both the well region and adjacent edge termination zone are disposed at an upper surface of the upper layer, and an oxide layer overlies the upper layer and the edge termination zone. A process for forming a silicon die having improved edge termination. The process comprises forming an upper layer comprising doped silicon of a first conduction type on a heavily doped silicon substrate, and forming an edge termination zone that comprises a layer of a material having a higher critical electric field than silicon at an upper surface of the upper layer. A well region of a second, opposite conduction type is formed at the upper surface of the upper layer adjacent the edge termination zone, and an oxide layer is formed over the upper layer and edge termination zone.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a divisional application of U.S. patentapplication Ser. No. 09/344,868, filed Jun. 28, 1999 (Attorney DocketNo. 87552.99R097/SE-1517PD).

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to silicon power semiconductordevices and, more particularly, to a silicon semiconductor die having anefficient and reliable edge termination zone.

BACKGROUND OF THE INVENTION

[0004] PN junctions within semiconductor devices are not infinite,terminating at the edge zones of a die This edge effect limits thedevice breakdown voltage below the ideal value, V_(brpp), that is set bythe infinite parallel plane junction. Care must be taken to ensureproper and efficient termination of the junction at the edge of the die;if the junction is poorly terminated, the device breakdown voltage canbe as low as 10-20% of the ideal case. Such severe degradation inbreakdown voltage can seriously compromise device design and lead toreduced current rating as well. In addition, an inefficient edgetermination makes a device unstable and unreliable if the device isoperated in a harsh environment or over a long period of time.

[0005] Various edge termination techniques have been developed,including, for example, field plate (FP), described in F. Conti and M.Conti, “Surface breakdown in silicon planar diodes equipped with fieldplate,” Solid State Electronics, Vol. 15, pp 93-105, the disclosure ofwhich is incorporated herein by reference. Another edge terminationapproach is field limiting rings (FLR), described in Kao and Wolley,“High voltage planar p-n junctions,” Proc. IEEE, 1965, Vol. 55, pp1409-1414, the disclosure of which is incorporated herein by reference.Further edge termination structures utilized variable lateral dopingconcentration (VLD), described in R. Stengl et al., “Variation oflateral doping as a field terminator for high-voltage power devices,IEEE Trans. Electron Devices, 1986, Vol. ED-33, No. 3, pp 426-428, andjunction termination extension (JTE), described in V. A. K. Temple,“Junction termination extension, a new technique for increasingavalanche breakdown voltage and controlling surface electric field inp-n junction,” IEEE International Electron Devices Meeting Digest, 1977Abstract 20.4, pp 423-426, the disclosures of which are incorporatedherein by reference.

[0006] The purpose of all these various techniques is to reduceelectron-hole avalanche generation by lowering the peak electric fieldstrength along the semiconductor surface and thereby shifting theavalanche breakdown location into the bulk of the device. To achievethis goal, the width of the edge termination zone (L_(edge)) has to beseveral times higher than the depletion width (W_(pp)) of theparallel-plane portion of the PN junction. For example, if L_(edge)=2.98W_(pp), 98.7% of V_(brpp) can be achieved when an “ideal edgetermination,” as described in Drabe and Sittig, “Theoreticalinvestigation of plane junction termination,” Solid State Electronics,1996, Vol. 3, No. 3, pp 323-328, the disclosure of which is incorporatedherein by reference, is used. In practice, a longer Ledge than thetheoretical value should be used to guarantee device reliability.However, it is very important to point out that, even with veryefficient edge termination, electron-hole impact generation at a rate ofabout 1×10¹⁸ pairs/cm³.s, still exists along the semiconductor surface.

SUMMARY OF THE INVENTION

[0007] A silicon semiconductor die of the present invention comprises aheavily doped silicon substrate and an upper layer comprising dopedsilicon of a first conduction type disposed on the substrate. The upperlayer comprises a well region of a second, opposite conduction typeadjacent an edge termination zone that comprises a layer of a materialhaving a higher critical electric field than silicon. Both the wellregion and adjacent edge termination zone are disposed at an uppersurface of the upper layer, and an oxide layer overlies the upper layerand the edge termination zone.

[0008] Further in accordance with the present invention is a process forforming a silicon die having improved edge termination. The processcomprises forming an upper layer comprising doped silicon of a firstconduction type on a heavily doped silicon substrate, and forming anedge termination zone that comprises a layer of a material having ahigher critical electric field than silicon at an upper surface of theupper layer. A well region of a second, opposite conduction type isformed at the upper surface of the upper layer adjacent the edgetermination zone, and an oxide layer is formed over the upper layer andedge termination zone.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009]FIG. 1 schematically illustrates the formation of an edge zone inan silicon die by implantation and diffusion.

[0010] FIGS. 2-4 schematically depict the formation of a silicon diehaving a silicon carbide edge zone of a selected thickness.

[0011]FIGS. 5 and 6 illustrate the leakage current and electron-holeimpact ionization generation contours at the onset of edge breakdown fora prior art die having a field plate.

[0012]FIGS. 7 and 8 depict the leakage current and electron-hole impactionization generation contours at the onset of edge breakdown for a dieof the present invention that includes a field plate.

[0013]FIGS. 9 and 10 illustrate the leakage current and electron-holeimpact ionization generation contours at the onset of edge breakdown fora second prior art die having a field plate, a thin oxide layer, and adeep P-well.

[0014]FIGS. 11 and 12 depict the leakage current and electron-holeimpact ionization generation contours at the onset of edge breakdown fora second embodiment of the present invention, wherein the die does notinclude a field plate.

[0015]FIG. 13 is a plot comparing electron-hole avalanche generationrates for silicon dies of the prior art and the present invention.

[0016]FIGS. 14A and 14B compare surface depletion layer boundaries indies of the prior art and the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0017] The present invention provides more efficient and reliable edgetermination for silicon power semiconductor devices compared withcurrently known techniques. In accordance with the invention, thesilicon within an edge zone of a silicon die is replaced with a materialhaving a higher critical electric field (E_(crit)), which is the maximumelectric field under breakdown condition, and a lower impact ionizationgeneration rate, which is the number of electron-hole pairs generated byan electron or a hole per unit distance traveled.

[0018] Replacement of silicon with a suitable material for this purposecan be accomplished in several ways, including, for example,implantation or deposition, or by heteroepitaxial growth, as described,for example, in Madapura et al., “Heteroepitaxial Growth of SiC on Si(100) and (111) by Chemical Vapor Deposition Using Trimethylsilane,”Journal of the Electrochemical Society, 1999, Vol. 46, No. 3, pp1197-1202, the disclosure of which is incorporated herein by reference.SiC, because of its high E_(crit) (˜12 times higher than Si) andcompatible thermal oxidation process with silicon, is a usefulreplacement material for silicon in a die edge zone. Possible processesto produce the SiC material edge to a controlled depth in a siliconsubstrate are illustrated in FIGS. 1-4.

[0019]FIG. 1 illustrates the implantation of carbon, C, into the edgezone 101 of a die 100, shown as an N-epitaxial layer, using an oxidemask 102. A high temperature process, such as laser-promoted localannealing, can be used to activate and diffuse the C implant, resultingin the edge surface layer 103 of silicon die 100 being converted to SiC.

[0020] FIGS. 2-4 illustrate another possible process to make a SiC dieedge zone. First, a recessed edge zone 201 is etched by either a dry ora wet etch procedure to a specified depth in a silicon die 200,represented as an N-epitaxial layer, using an oxide mask 202 to preventsilicon removal from the active region 203, as shown in FIG. 2. A SiClayer 204, which is of the same conduction type as the epitaxial layer,is formed on recessed edge zone 201 using heteroepitaxial growth ordeposition techniques, as shown in FIG. 3. Oxide mask 202 is thenremoved, and chemical-mechanical polishing (CMP) is performed to producea fully planarized die 205 that includes SiC edge zone 204 a having aselected thickness, as demonstrated in FIG. 4.

[0021] Computer simulations have been performed to verify the conceptfor a wide range of breakdown voltages, from 600V to 30V. Simulatedleakage current and electron-hole impact ionization generation contours,511 and 508, respectively, at the onset of edge breakdown for a priorart die 500 with field plate edge termination are shown in FIGS. 5 and6, respectively. Die 500 includes a substrate 501 bearing an N-epitaxiallayer 502, in which is implanted a P-well 503. On the surface 504 ofepitaxial layer 502 is deposited an oxide layer 505 and, in contact withP-well 503, a front metal layer 506 that is further in contact with afield plate 507. A back metal layer (not shown) is formed on the bottomof substrate 501.

[0022] As shown in FIG. 6, the highest electron-hole generation site 508is located close to the intersection of the PN junction 509 betweenP-well 503 and N-epitaxial layer 502 and silicon upper surface 504. Thisis due to the termination of PN junction 509 to form a planar diffusedjunction having a finite curvature, which causes electric field crowdingnear upper surface 504 and leads to large impact ionization values atthe die edges. Consequently, the breakdown occurs at junctiontermination edge 508 rather than in the parallel plane portion 510.

[0023] The breakdown characteristics of a die 700 of the presentinvention having a field plate edge and a SiC edge zone formed by Cimplantation and diffusion have also been simulated. Die 700,schematically depicted in FIGS. 7 and 8, includes a substrate 701bearing an N-epitaxial layer 702, in which is implanted a P-well 703. Atthe surface 704 of epitaxial layer 702 is formed a SiC edge zone 705. Anoxide layer 706 is formed on SiC edge zone 705, and a front metal layer707 interconnects P-well 703 with a field plate 708. A back metal layer(not shown) is formed on the bottom of substrate 701.

[0024]FIGS. 7 and 8 illustrate the simulated avalanche leakage currentand impact ionization contours, 709 and 710, respectively, for die 700.The breakdown location 710 is completely screened from the oxide layer706 by SiC edge zone 705, and there is very little electron-holegeneration along upper surface 704. The breakdown voltage for die 700 ishigher than that observed for die 500.

[0025] In order to reduce the electron-hole avalanche generation ratealong the Si/oxide interface and improve device reliability, a prior artdie 900 makes use of a deeper PN junction and thinner oxide to lower thecurvature effect. Die 900 with field plate edge termination includes asubstrate 901 bearing an N-epitaxial layer 902, in which is implanted adeep P-well 903. On the surface 904 of epitaxial layer 902 is depositeda thin oxide layer 905 and, in contact with P-well 903, a front metallayer 906 that is further in contact with a field plate 907. A backmetal layer (not shown) is formed on the bottom of substrate 901.

[0026]FIGS. 9 and 10 illustrate the simulated avalanche leakage currentand impact ionization contours, 908 and 911, respectively, for prior artdie 900. By properly choosing the depth of PN junction 909 and thethickness of oxide layer 905, the breakdown location is moved to theparallel plane portion 910 of PN junction 909. As a result, the devicereliability of die 900 can be substantially improved. However, althoughthe avalanche breakdown location is shifted into the bulk silicon, therestill exists a certain level of electron-hole generation along theinterface 904 between epitaxial layer 902 and oxide layer 905. Thesimulation gives an impact ionization generation rate at breakdownlocation 911 of about 1×10²¹ pairs/cm³.s and a generation rate at thesame voltage of about 1×10¹⁸ pairs/cm³.s at surface 904.

[0027] The present invention provides further improvement, without theneed for changing junction depth and oxide thickness, over the resultsobtained with prior art die 900. Die 1100, schematically depicted inFIGS. 11 and 12, includes a substrate 1101 bearing an N-epitaxial layer1102, in which is implanted a P-well 1103. At the surface 1104 ofepitaxial layer 1102 is formed a SiC edge zone 1105 that extends intoN-epitaxial layer 1102 to a depth below that of P-well 1103. An oxidelayer 1106 is formed on SiC edge zone 1105, and a front metal layer 1107interconnects P-well 1103. Unlike previously described dies, die 1100includes no field plate. A back metal layer (not shown) is formed on thebottom of substrate 1101.

[0028] By making the SiC edge layer deeper than the planar PN junction,edge termination with the ideal breakdown voltage can be achieved.Furthermore, the field plate can be omitted without degrading the devicebreakdown characteristics. FIGS. 11 and 12 illustrate the simulatedavalanche leakage current and impact ionization contours, 1110 and 1111,respectively, for die 1100 of the present invention. The breakdownlocation 1108 is optimally situated at P-N junction parallel planeportion 1109. In addition, the electron-hole generation at upper surface1104 is extremely low.

[0029]FIG. 13, a plot of impact ionization along the Si/SiO₂ interfaceversus distance from the P-N junction at the interface, depicts thesurface carrier generation characteristics of the field plate-containingprior art dies 500 (cf FIGS. 5,6) and 900 (cf. FIGS. 9,10) shown inFIGS. 5 and 9, along with die 1100 (cf. FIGS. 11, 12) of the presentinvention. The SiC edge termination included in die 1100 lowers theelectron-hole avalanche generation rate more than 20 orders of magnitudecompared with prior art die 500, and more than 16 orders of magnitudecompared with prior art die 900. Furthermore, the breakdown voltage ofdie 1100 is desirably increased as a result of the thicker net epitaxiallayer, which is defined by the distance between the parallel planeportion 1109 of the PN junction and highly doped substrate 1101.

[0030] Another improvement provided by the present invention is areduction in edge termination area, which is controlled by the width ofthe surface depletion layer. Edge termination in accordance with thepresent invention does not change the curvature of the edge planarjunction and the equal-potential contour distributions. Therefore thewidth of the surface depletion layer, which is less than the depletionwidth of the parallel plane portion, does change. According to theanalysis described in the previously mentioned paper of Drabe andSittig, the area of edge termination in die 1100 is expected to be abouthalf that of the theoretical “ideal” Si edge termination.

[0031] In the absence of any termination structures, the width of theedge zone containing material with a higher critical electrical fieldthan silicon can be chosen to be equal to the width of the surfacedepletion layer of the edge planar junction. To verify this, the widthof the SiC edge zone 1105 in die 1100 (cf. FIG. 12) is reduced tocorrespond to the surface depletion layer boundary 1112 of the PNjunction 1109. The simulated breakdown characteristic does not change,and the breakdown voltage also remains the same. The depletion layerboundary 1112 of die 1100 at the onset of avalanche breakdown is shownin FIG. 14A. The depletion layer boundary 912 of field plate-containingdie 900 (cf FIG. 10) is depicted in FIG. 14B. The width of the depletionlayer in prior art die 900 is at least two times greater than that ofdie 1100 of the present invention.

[0032] In addition to the described field plate (FP), the edgetermination of the present invention can be advantageously applied insemiconductor dies that include other edge terminating features such as,for example, field limiting rings (FLR), variable lateral dopingconcentration (VLD), and junction termination extension (JTE).

[0033] The present invention has been described in detail for thepurpose of illustration, but it is understood that such detail isstrictly for that purpose, and variations can be made therein by thoseskilled in the art without departing from the spirit and scope of theinvention, which is defined by the following claims.

What is claimed:
 1. A silicon semiconductor die comprising: a heavilydoped silicon substrate, an upper layer comprising doped silicon of afirst conduction type disposed on said substrate, said upper layercomprising a well region of a second, opposite conduction type adjacentan edge termination zone, said well region and said adjacent edgetermination zone both being disposed at an upper surface of said upperlayer; and an oxide layer overlying said upper layer and said edgetermination zone; wherein said edge termination zone comprises a layerof a material having a higher critical electric field than silicon. 2.The die of claim 1 wherein said upper layer is an epitaxial layer. 3.The die of claim 1 wherein said first conduction type is N and saidsecond conduction type is P.
 4. The die of claim 1 wherein said firstconduction type is P and said second conduction type is N.
 5. The die ofclaim 1 wherein said edge termination zone comprises a layer of siliconcarbide.
 6. The die of claim 5 wherein said layer of silicon carbide isformed by implantation, activation, and diffusion of carbon into saidupper silicon layer.
 7. The die of claim 5 wherein said layer of siliconcarbide is formed by deposition .
 8. The die of claim 5 wherein saidlayer of silicon carbide is formed by 103709 -8heteroepitaxial growth.9. The die of claim 1 further comprising a front metal layer overlyingand in electrical contact with said well region and a back metal layerdisposed on a bottom surface of said substrate.
 10. The die of claim 9further comprising a field plate in electrical contact with said frontmetal layer.
 11. The die of claim 1 wherein said edge termination zonehas a selected thickness.
 12. The die of claim 11 wherein said edgetermination zone is recessed in said upper layer and extends into saidupper layer to a depth exceeding the depth of the adjacent well region.13. The die of claim 1 further including a field plate.
 14. The die ofclaim 1 further including field limiting rings.
 15. The die of claim 1further including variable lateral doping concentration.
 16. The die ofclaim 1 further including junction termination extension.
 17. A processfor forming a silicon die having improved edge termination, said processcomprising: forming an upper layer comprising doped silicon of a firstconduction type on a heavily doped silicon substrate; forming an edgetermination zone at an upper surface of said upper layer, said edgetermination zone comprising a layer of a material having a highercritical electric field than silicon; forming a well region of a second,opposite conduction type in said upper layer adjacent said edgetermination zone; and forming an oxide layer over said upper layer andsaid edge termination zone.
 18. The process of claim 17 wherein saidupper layer is an epitaxial layer.
 19. The process of claim 17 whereinsaid first conduction type is N and said second conduction type is P.20. The process of claim 17 wherein said first conduction type is P andsaid second conduction type is N.
 21. The process of claim 17 whereinsaid edge termination zone has a selected thickness.
 22. The process ofclaim 17 wherein said edge termination zone comprises a layer of siliconcarbide.
 23. The process of claim 22 wherein said forming said layer ofsilicon carbide comprises implanting, activating, and diffusing carboninto said upper silicon layer.
 24. The process of claim 22 wherein saidforming said edge termination zone comprises etching said upper surfaceof said upper layer prior to forming said layer of silicon carbide,thereby providing an edge termination zone recessed in said upper layer.25. The process of claim 24 wherein said edge termination zone recessedin said upper layer extends into said upper layer to a depth exceedingthe depth of the adjacent well region.